This proposal addresses the control of cellular differention in neuronal progenitor cells during brain development. The mammalian neocortex is composed of numerous subpopulations of neurons with characteristic morphologies, patterns of connectivity and functions. These subpopulations arise from a seemingly homogeneous population of proliferating cells in the neuroepithelial layer lining the lateral ventricles of the brain. Evidence suggests that cell-intrinsic determinants play a role in restricting the potency of cells as they exit mitosis and begin to migrate toward their final cortical destination. Laser capture microdissection or cell sorting by flow cytometry will be used to isolate neuronal precursors that have just exited mitosis, and the RNA that is extracted from these cells will be screened by degenerate RT-PCR and cDNA microarrays to identify genes that are differentially expressed in cells that exited mitosis earlier in embryonic development versus later. In addition, we will use ectopic expression studies and deletion analysis to characterize a novel gene previously identified in the laboratory, K12, which is differentially expressed in lymphocyte precursors and in all neuronal precursors following exit from mitosis. Thus, the studies proposed will contribute towards our understanding of the cellular mechanisms important for the cessation of proliferation and the initiation of a cellular differentiation program in the developing neocortex and in general, an aspect of cell behavior that, when uncontrolled, contributes to the development of cancer.